Soundings

Building on Ledge

Q:

We are
considering using foam forms for a stepped foundation on a
steep site overlooking the coast. The tricky part is that the
site is mostly granite ledge. Can the concrete in the forms be
pinned to the ledge? If so, how would one go about scribing the
forms to fit the uneven surface?

A:

Builder Phil Harrison of
Portland, Maine, responds: It's important to bring in an
engineer early on such a project. How you pin a foundation to
ledge depends on the strength of the rock to hold the
anticipated loads. If you have hard, stable rock, it's possible
to hold the foundation with steel dowels. If the rock is loose
or shattered, however, it may be necessary to blast down to
stable material or drill through the rock for piers. It can get
complicated very quickly, particularly on cliff-side
sites.

Most of the sites we have built on around Casco Bay have had a
solid granite ledge beneath thin patches of "blueberry sod." We
have to remove the sod entirely, but the rock itself is stable
enough to hold epoxy-set pins, allowing us to pin the poured
concrete walls without additional footings or pier bearings.
While the uneven surface of the rock provides a good key for
the concrete, we typically must use dowels cut from No. 6
(3/4-inch-diameter) rebar. These are usually set 6 to 8 inches
into the rock and spaced every 2 feet, depending on the contour
of the bearing surface and the wall elevation. The engineer may
require a closer spacing near corners or where the foundation
radically changes elevation.

We've used foam forms for several poured foundations on ledge.
They make sense for a number of reasons. For starters, I feel
carpenters are usually better at squaring, plumbing, and
leveling than are concrete formers. Also, foam is much easier
to cut to fit the complex contour of the rock than are wood
forms. And in our area, R-6 insulation on the foundation walls
is necessary for the thermal performance of the building, so it
makes sense to form and insulate in one step.

Scribe to Fit

The tricky part, as you seem to understand, is the layout and
scribing process. We've done several foundations this way over
the years, and we've yet to discover any real shortcut.

We first lay out the corners of the building. Typically, we'll
contract with the engineer for this, particularly if the site
is very steep. Using a theodolite, a well-trained survey crew
can accomplish in an hour what it would take me most of a day
with my builder's level, and I wouldn't ever feel very certain
of my results. The survey crew gives us clear corners
spray-painted on the rock. We can then go to work setting our
pins.

To start, we simply stretch a line from the building corners
and spray-painted reference marks so we can eyeball the edges
of our concrete walls. Then we go to work on the laborious task
of drilling for the foundation pins in the center of the wall
section. It's not unusual to have 150 pins in a house
foundation, so it's nearly a full day for two people just to
drill and set the dowels in epoxy.

Once the pins are placed and capped off for safety, we drill
additional holes outside the building lines for rebar stakes to
support batter boards, and then we string level lines. This is
a bit tedious, but once the batter boards are in place and the
strings up, we have an essential reference from which to
measure to get a rough idea of the difference in elevation for
scribing the foam blocks. We rough-cut the blocks with a
folding tree saw to match this first rough calculation, then
fine-tune the scribe holding a 4-foot level on top of each
block. It's slow going, but it ends up fairly accurate. Using
this system, we've had good luck avoiding blowouts when pumping
the concrete.

Our success at avoiding blowouts can also be attributed to
rigorous bracing (we'd rather overbrace than risk a blowout),
to always using a low (3-inch) slump concrete, and to taking
our time to pump slowly, distributing the concrete evenly. We
also make good use of a hand-held concrete vibrator to
eliminate voids inside the formwork.

Drainage Details

The most challenging part of building a foundation on ledge is
working out an effective drainage plan. Water tends to flow
along the ledge, and it can move through fissures in the ledge,
so it's not uncommon to have a spring flowing out of a rock
outcropping inside the basement. There's no stopping this water
flow. The best steps you can take are to contain it and to
divert it outside. We typically run perimeter drains inside and
outside the foundation walls. Inside the foundation, we dump
yards and yards of compacted gravel over the drain lines to
level out for a slab. The inside lines are connected to the
exterior lines at each low point in a stepped section. Backflow
valves at these connection points prevent backed-up lines from
overflowing into the foundation.

Because EPS foam will absorb water, I am concerned that the
forms might become waterlogged in the vigorous runoff that
occurs each spring. We waterproof with a self-adhering
rubberized asphalt membrane, applying the membrane wherever the
foundation will be backfilled, even on the inside if a
significant heap of gravel fill bears against the
foundation.

Erosion of Light-Commercial
Property

Q:

I am a manager in
St. Augustine, Fla., of a property constructed in 1983. We have
a lower parking structure that has suffered erosion because of
poor drainage from an upper parking structure (the outlet is
simply a square hole). This erosion has washed away the soil
below the stucco, leaving about 3 inches of concrete exposed
above grade. Which would be the better solution: Haul in dirt
to fill the gap and bring the grade up to the stucco line, or
bring in 1 to 2 inches of river-rock stone to enable runoff
into the drainage ditch that runs horizontally 10 to 12 feet
away? Is it true that the concrete being exposed, without
stucco on it, makes this lower parking structure more
susceptible to cracking?

A:

Charlie Gardner, a
Philadelphia-based engineer specializing in storm water
management, drainage, and hydraulic problems, responds:
When concentrated rainwater falls through air from a roof, it
picks up a lot of energy. This energy is showing up at the
wrong place on your site, causing long-term erosion, increased
foundation exposure, and appearance problems. As a practical
matter, think of your problem as requiring a two-part solution:
roofing work and work at grade level.

Roofing work first. You mention a
square hole through which drainage is discharged from an upper
parking structure. It sounds as if you need a downspout sized
to handle the runoff. The rate of discharge will be
proportional to the area drained. Delegate the calculations and
code compliance to your roofer, who should be able to size the
rainwater conductor (RWC) according to the applicable local
codes or BOCA for the amount of contributing roof area.

A cast-iron or hot-dip galvanized steel collector box with
galvanized steel pipe would be used on an elevated highway
structure, but this is probably overkill for your application.
Regardless, have your roofer show you a picture of the
collector box he plans to use and describe how he would hang
it. It should be noncorrodible aluminum or plastic. If a
plastic RWC is to be used, specify at least Schedule 40 PVC to
prevent incidental mechanical damage from minor impacts.

To install it, construct from the roof down. The collector box
should be somewhat larger than the aperture in the roof slab to
catch all of its discharge. I would try to mount it with an air
gap and a 1/4-inch mesh hardware screen cover to keep out bird
nests. The box should be connected with the RWC to the exterior
side of a wall and down, terminating in an outward-bending
elbow above grade level. You will want the parts to be well
fastened, and probably you should consider tamperproof
connections.

Every change of direction takes energy away from moving water.
Flow friction next to surfaces also takes energy away from
moving water. With the RWC, you obtain two benefits: You have
made the energy problem shrink, and you now have the wild water
under control.

The work at grade level. Your letter
asks about the desirability of bringing in dirt to raise the
grade at the building. This is your decision. Keep in mind that
grades adjacent to a building should always slope away from the
walls. Life would be simple if all buildings were on hilltops.
Regardless, you are fortunate in having a ditch 10 to 12 feet
away into which you can discharge rainwater. The bottom (we
call the lowest point in the ditch cross-section the invert)
should be well below the soil line on the wall. If this is not
the case, you should either lower the ditch invert or raise the
fill against the wall, or do both.

If the work is to be done by one contractor, he will take
responsibility for the fitting and matching of parts. If done
by different workmen, mark your proposed soil line at the wall
and point it out to the roofer so he can allow for the splash
block.

It still remains to dissipate the kinetic energy left in the
water that will come shooting out. This will only happen during
the kind of rain that keeps people indoors, but it will happen.
Here you have several choices and even some room for
creativity. A large splash block should tilt away from the
building. Then armor the flow path to the ditch. I have seen
the river rock you mentioned used very attractively. There are
several other materials that could be used, including
landscaping possibilities. The objective should be to clear the
foundation zone and reduce velocity to prevent erosion as the
water flows toward the ditch.

Stucco repair. As to the lack of
stucco causing cracks in structural concrete, I am somewhat
skeptical. Concrete does crack for a large number of reasons,
but to my knowledge, lack of stucco is not one of them. Even
though well-applied stucco is a quality product, the icing is
not what holds the cake together. If the issue is cosmetic,
consider reapplying stucco, or even a thorough cleaning
followed by a good coat of masonry paint.

Although you did not mention seepage through the wall, the
interior lower-level garage will be drier after the new RWC is
installed. After all, the best way to waterproof a wall is to
not charge the soil behind it with water in the first
place.